Electrolytic capacitors are generally the capacitors of choice for applications demanding high capacitance/size and high capacitance/cost ratios. These devices exploit the relatively high dielectric constants and very high withstanding voltages per unit thickness which may be obtained in the anodic oxide films grown upon valve metals with appropriate electrolytes and anodizing conditions.
Aluminum is typically employed as the valve metal in electrolytic capacitors where low cost per unit capacitance is a primary concern, while tantalum is usually employed for applications in which high reliability and volumetric efficiency (i.e., high capacitance per unit volume) are the primary concerns. Other valve metals, including niobium, titanium, and titanium aluminides have also been used to construct capacitors, but aluminum and tantalum remain the main materials of choice. The valve metal may be utilized in the form of an etched foil or as a porous, sintered powder-metallurgy compact at significantly below theoretical density. In either form, the valve metal is anodized to produce the anodic dielectric film prior to application of the cathode material.
Electrolytic capacitors may be classified by the type of cathode material employed in their construction. So-called "wet" capacitors contain aluminum or other valve metal foil immersed in a container of liquid electrolyte. Although these devices were once commonplace, their low volumetric efficiency has rendered them obsolete. "Wet slug" tantalum capacitors in which sintered, powder metallurgy, anodized tantalum anodes are immersed in a minimum volume of highly conductive liquid electrolyte (e.g. 40% sulfuric acid) find application where high volumetric efficiency and capacitance combined with high reliability are desired. These devices tend to have relatively high ESR due to the resistivity inherent in ionically conductive liquids. In the event of seal failure, the acid and concentrated salt electrolytes employed in their construction tend to be very corrosive to circuit boards and other components.
So-called "dry" electrolytic capacitors were developed to increase the volumetric efficiency of "wet" aluminum capacitors while avoiding the highly corrosive electrolytes and additional expense associated with "Wet slug" tantalum capacitors. This type of capacitor has one or more anode and cathode foils separated by highly absorbent paper. The foil/paper combination is wound to form a cylinder having protruding tabs for electrical connection, and this cylinder assembly is then impregnated with a liquid electrolyte prior to assembly into a case which surrounds the impregnated assembly.
The "wet," "wet slug," and "dry" capacitor constructions all have in common the presence of a liquid electrolyte in contact with the valve metal anode. Any cracks or defects in the anodic oxide (due to the capacitor assembly process, etc.) may be at least partially healed during use by the application of voltage, which results in the growth of fresh anodic oxide or the isolation of flaws by the presence of gas bubbles from electrolysis of the electrolyte.
In the 1950's, a new type of electrolytic capacitor was introduced in which the cathode material is a true "solid" These devices usually contain a sintered, powder metallurgy anodized tantalum anode which has been impregnated with manganese dioxide via pyrolysis of manganese nitrate solution. More recently, "solid" electrolytic capacitors have been introduced which employ intrinsically conductive polymers, such as polypyrrole, polythiophenes, etc., as the cathode materials.
The introduction of conductive polymer cathode materials has facilitated the use of aluminum and other valve metals in addition to tantalum in "solid" capacitors due to the elimination of the multiple pyrolysis steps at the relatively high temperatures (200-400.degree. C.) required to produce manganese dioxide cathode material within the pore structures of anodes after first impregnating the anode bodies with manganese nitrate solution.
The construction of "solid" electrolytic capacitors eliminates the contact to the anodic oxide by a liquid electrolyte. The absence of a liquid electrolyte minimizes the amount of dielectric flaw "healing" or isolation which can be accomplished in the finished device due to extreme heating of the oxide at flaw sites brought about by the higher currents supported by manganese dioxide or conductive polymers compared with the more resistive liquid electrolytes. The elimination of liquid electrolyte also minimizes the heat sink action of the cathode material at flaw sites (localized boiling of liquid electrolyte tends to carry heat away from flaw sites).
In order to overcome the difficulty of repairing flaws in the anodic oxide dielectric of assembled "solid" electrolytic capacitors, one or more electrolytic treatment steps (known as "reformation" or, simply, "reform" steps; the initial anodization which produces the anodic oxide is known as the "formation" step(s)) are carried out in which the anode bodies containing manganese dioxide or conductive polymeric material are immersed in a liquid electrolyte and a positive voltage is applied to the anode bodies while a negative voltage is applied to the electrolyte. The voltage applied to the anodes is generally lower than that used to produce the anodic oxide, so that the vast majority of any current flowing through the anode bodies flows through the flaw sites. This current flow is thought to repair the flaws by the growth of new oxide at the flaw or, especially, by thermally and electrochemically degrading the cathode material locally, thereby isolating the flaw sites electrically. Reformation electrolytes generally contain a small amount of phosphoric acid as the ionogen. Although many other ionogens have been employed in "reform" electrolytes including sulfuric acid, nitric acid, acetic acid, and sulfosalicylic acid, the presence of the orthophosphate ion has generally been found to give the best results with respect to the leakage current of the finished devices.
As stated above, the liquid electrolyte used for reformation serves as a heat sink to prevent run-away heating at flaws and the resistivity of the electrolyte serves to act as a resistor in series with each capacitor anode, limiting the current and the resulting current-driven heating of the flaws during the reform process.
Due to the current-limiting aspect of the reform electrolyte the resistivity of this electrolyte is usually carefully controlled. The optimal resistivity range for reform electrolytes depends upon the applied voltage, electrolyte temperature, and the chemical nature of the cathode material involved. What is generally desirable, however, is minimal resistivity change during use.
Unfortunately, anodes which have been impregnated with solid cathode materials frequently contain ionic materials which leach into the reformation electrolyte during the reformation step(s). Manganese dioxide containing anodes tend to contain nitrogen oxides adsorbed on the high surface area manganese dioxide, as well as a small amount of unreacted manganese nitrate. Organic polymer containing anodes tend to contain a certain amount of uncombined dopant acid, such as toluene sulfonic acid. It has proven to be very difficult to reduce the level of these ionic contaminants to the degree that they do not result in resistivity depression of the reformation electrolyte; even when anodes are exposed to prolonged hot de-ionized water rinsing prior to the reform steps, some ionogens are released by the electrochemical action.
In a manufacturing environment, it is highly desirable to reduce the resistivity depression of the reformation electrolyte so as to avoid the necessity of frequent changes of the electrolyte. Traditionally, this problem has been addressed by the use of aqueous phosphoric acid solutions containing a substantial percentage of ethylene glycol. The glycol acts to raise the resistivity of the electrolyte for a given ionogen content and temperature. As the electrolyte becomes contaminated by ionogens from the solid impregnated anodes, the ethylene glycol content has been progressively increased in order to maintain the resistivity within specified limits. Thus the reformation electrolyte may be used for a significantly larger number of anodes prior to replacement, thereby facilitating greater manufacturing throughput per tank of reformation electrolyte.